1. Field of the Invention
The present invention relates to an optical head device for protecting and recording data on optical recording media such as compact disks (CDs) and digital video disks (DVDs) that have different substrate thickness by using laser beams having different wavelengths. Moreover, the present invention relates to objective lenses that are suitable for such optical head device.
2. Related Background Art
Optical recording media having transparent substrates for protecting recording surfaces with different thickness and different recording density such as CDs and DVDs are known. For example, each CD is provided with a transparent substrate for protecting its recording surface whose thickness is 1.2 mm, while each DVD is provided with a transparent substrate that is thinner than that of the CD, which is 0.6 mm, and has a higher recording density than that of the CD.
There have been proposed optical head devices that record and reproduce information on optical recording media, which use, in order to make the optical head devices smaller and more compact, a single objective lens to focus laser beams on recording surfaces of CDs and DVDs.
Such an objective lens may have a lens surface having a single refractive power that is provided with a minute diffraction grating. An incident beam is diffracted by the diffraction grating, such that a plurality of focal points are formed at different positions along an optical axis with diffracted beams of multiple orders. Also, the diffraction property of the diffraction grating is designed such that high light converging efficiencies are attained at two focal points among the focal points formed with the multiple diffracted beams of multiple orders, and one of the diffracted beams forming these two focal points on a high order side is focused at a position with a shorter focal length.
An optical head device using the objective lens described above diffracts a laser beam with a wavelength of 650 nm into two diffracted beams, for example, a first order diffracted beam and a second order diffracted beam such that the first order diffracted beam can be focused at a first focal point on the recording surface of a CD, and the second order diffracted beam can be focused at a second focal point with a shorter focal distance on the recording surface of a DVD.
However, since the objective lens described above is designed on the premise that a laser beam with a single wavelength is converged at two different focal points, this objective lens is not suitable for a two-wavelength light source type optical head device that uses laser beams with different wavelengths, such as an optical head device equipped with a light source for a laser beam with a wavelength of 780 nm to be used for reproducing and recording data on a compact disk recordable (CD-R) and a light source for a laser beam with a wavelength of 650 nm to be used for reproducing data on a DVD.
A two-wavelength light source typo optical head device may use an objective lens provided with a lens surface with three split faces that are concentric from an optical axis side to an outer circumferential side of the objective lens, i.e., a first split face of a circular shape, a second split face of a ring-shape and a third split face of a ring-shape. Also, the second split face is formed with a diffraction grating.
The objective lens described above converges a beam that passed the first split face and a 0th order beam that passed the diffraction grating of the second split face at a first focal point for reproducing and recording data on a CD-R in the case of the laser beam with a wavelength of 780 nm, and converges a beam that passed the first and third split faces and a 1st order diffracted light that has been diffracted by the second split face at a second focal point for reproducing data on a DVD. For this reason, the first split face through the second split face are used as an aperture for CD-Rs, and the first split face through the third split face are used as an aperture for DVDs.
However, despite the fact that two types of media having different substrate thickness are subject to the objective lens described above, the lens surface, which is a basis of the objective lens, is split into three faces. This forces a severe burden on designing the diffractive grating, and invites deterioration of the light utility efficiency.